The present invention relates to a mechanism which mimicks the illumination of the Moon as it passes through its various phases.
The Moon has a significant physical influence on life on our planet due to its large size and its close proximity. At full moon it can reflect sufficient sunlight to light up the night. Its mass is great enough to distort the Earths shape and to produce tides in oceans and lakes. It also provides the main force that moves the poles of the Earth in the precession of the equinoxes. Its shadow on the earth at occasional times and places may obscure light from the Sun to produce solar eclipses. Though these influences on Earth may be subject to simple laws of physics and dynamics, we are only just beginning to understand how the Moon may influence the biology of life on our planet.
The calendar month is equivalent approximately to the period of revolution of the Moon around the Earth. This period (29 days 12 hours 44 minutes 2.8 seconds) is the synodic month and represents the time it takes for the Moon to pass through the sequence of phases from new to first quarter to full to third quarter to new again and make a complete revolution about the Earth with respect to the Sun.
The relative positions of the Sun, Earth and Moon affect the illuminated lunar image that is seen by an observer. The xe2x80x9cNewxe2x80x9d phase occurs when the moon surface is in full shade and all three bodies are linearly aligned with the Moon positioned centrally. The phase xe2x80x9cFirst Quarterxe2x80x9d occurs when the half moon surface is in sunlight forming a semi-circular shape. This occurs as the moon revolves around the Earth with the Sun Earth Moon angle describing an approximate right angle. The phase xe2x80x9cFullxe2x80x9d occurs when the observed surface of the moon is fully illuminated by the Sun and again all bodies are linearly aligned but with the Earth centrally positioned. The phase xe2x80x9cThird or last quarterxe2x80x9d relates to a xe2x80x9chalt moonxe2x80x9d but this time the moon position is between the new moon and full moon (ie approximately one hundred and eighty degrees removed from the first quarter). The left hand side of the Moon is illuminated as observed from Earth whereas it is the right hand side which is illuminated at the first quarter. In between these points, the shape of the moon appears as illuminated crescents or is gibbous and various angles of tilt can be observed.
Conventional moon dials (as used on various moon-phase clocks and watches) may comprise a single disc printed with two circular moon shapes positioned at 180 degrees to one another rotating behind a shaped window whose shape masks the visible or partly visible moon as it rotates to give an impression of the lunar phase. The window shape is similar to an axehead positioned with the crescent xe2x80x9ccutting edgexe2x80x9d uppermost and semicircular convex and concave aides which represent shadow as the moon disc rotates clockwise west to east. A disadvantage of this mechanism is that, as the moon goes from the third quarter to full phase, the shape of the shadow does not accurately represent the shadow which is observed on a near spherical object such as the Moon. As the Moon is gibbous and approaches fullness the pattern of the illuminated Moon produced by this mechanism is still crescent-like whereas it is the shadow which should be crescent shaped. Similarly, as the full Moon phase ages further, the initial shadow effect produced by the Eastern semicircular edge of the window is incorrect in that the appearance of the Moon should only become crescent shaped after the shadow covers more than half of the visible surface of the moon.
A second conventional mechanism utilises a rotating globe, halt of which is painted white and the other half black. As this rotates, an accurate impression of lunar phase is displayed but the size of the sphere is a significant disadvantage.
A third conventional device is one in which the various phases of the moon are separately displayed around the circumference of a clock and a pointer indicates the particular phase at a given time. Again this method suffers the disadvantage relating to the size of the moon, since many separate moons have to be represented on the same dial.
The present invention seeks to overcome these disadvantages by providing a stationary lunar image which mimmicks the appearance of the actual lunar phase more closely. The invention allows the phase of the Moon to be represented more accurately for any day of the month.
Thus viewed from one aspect the present invention provides a moon-phase dial mechanism comprising:
a substantially circular window capable of exhibiting a visible representation of each of the phases of the moon in a lunar cycle; to the rear of the substantially circular window, a rotatable upper disc having a transparent area, a light area and a dark area; and
to the rear of the rotatable upper disc, a rotatable lower disc having a light area and a dark area,
wherein the upper and lower disc overlap so that the substantially circular window exhibits a visible representation of a phase of the moon being a combination of the light area and the dark area of the upper disc and the light area and the dark area of the lower disc exposed through the transparent area of the upper disc.
In accordance with the invention, as the upper and lower discs rotate, each of the phases of the moon in a lunar cycle are visible through the substantially circular window. The shading of each of the upper and lower discs and their alignment may be determined empirically and one example for illustrative purposes only is shown in the Figures attached hereto and described hereinafter.
In a preferred embodiment, the moon-phase dial mechanism further comprises: a back screen to the rear of the rotatable lower disc. Preferably the overlapping upper and lower disc are each fixed onto a separate retaining means (eg a central axle, one or more retaining pillars or a retaining ring) attached to the back screen. The upper disc may be positioned above the lower disc so as to overlap by an amount slightly less than a radius.
The rotatable upper and lower discs may be rotatably driven by any known suitable driving means. Such driving means may be capable of mechanically cooperating with said discs eg one or more cogs, gears, cams and the like.
Preferably each of the upper and lower disc is fitted rearwardly with at least one disc cog. For example, each disc cog may be mounted on separate retaining means (eg a central axle). In this arrangement, each of the upper and lower disc is conveniently driven in the same direction by a single driving cog which co-operates with each disc cog.
Preferably the moon-phase dial mechanism comprises a display screen incorporating the substantially circular window. Preferably the display screen is substantially the same shade as the dark shading on the upper disc and on the lower disc.
In a particularly preferred embodiment, the display screen further comprises an arc window and an edge of the upper or lower disc is printed with indicators of the days of the month (eg Arabic or Roman numerals or lettering), the arrangement being such that the arc window is capable of exhibiting each of the indicators of the days of the month. For example, the upper disc may have printed around the circumference the numbers 1-31 representing the days of the month.
The appropriate spacing of the indicators of the days of the month may be readily determined empirically so as to match the visible representation of the phase of the moon exhibited by the substantially circular window. For example, the circumference of the upper disc may be empirically calibrated using the known days of easily recognisable phases (eg full and new moon phases). Intermediate days of the month may be equally spaced between the selected calibrated days of the month.
If desired, the day of the month may be matched to the calendar month itself which may be printed on the display screen at the appropriate position alongside the arc window. The actual position of the printed calendar month has to take into account the difference between the lunar cycle and the variable number of days of each calendar month. The variable length of the calendar month may be allowed for in the simplest form by empirically determining the correct position of the calendar month labels on the display screen. Alternatively, the mechanism could be fitted with a series of gears to compensate for the difference and additional gears. A similar adjustment could enable the year to be indicated.
In one embodiment of the invention, the moon-phase dial movement of the invention is used as a lunar display on watches or clocks.
Thus viewed from a further aspect the present invention provides a time-piece (eg a clock or watch) comprising a moon-phase dial mechanism as hereinbefore defined together with a time-piece movement.
A series of cogwheels and gears may be utilised to pair the watch or clock movement with the moon-phase dial mechanism of the invention. As an example, a cog on a clock movement whose period is one day can drive a series of cogs whose end result is a gear ratio of about 1:29:5306 so that the overlapping dials rotation is equivalent to the synodic month.